The neotectonics of the Ruahine and Mohaka faults, between the Manawatu Gorge and Puketitiri; and figures to accompany final progress

Technical Abstract

New Zealand lies across the active IndoAustralian-Pacific plate boundary. Volcanic activity and earthquakes are a direct result of interaction between these two plates. The Wellington and Ruahine Faults are major strike-slip faults that have formed due to this interaction. These faults move sideways in a horizontal motion so that the area west of, and including the Central Ranges is moving northward while the East Coast of the southern North Island is moving southward. If earthquake activity is averaged out on a yearly basis, these two sections of the North Island move past each other at a rate of 5 to 7mm (for the Wellington area) and up to 15mm per year for the Dannevirke area. These rates of motion are high by world standards.

The purpose of this study was to establish a record of past earthquake events which would allow future estimates of fault behaviour to be made in the region between the Manawatu gorge and the Napier-Taupo highway. Trenches have been excavated through these faults in mainly swampy environments. Within these trenches are layers of earthquake debris, deformation and layers of peat. The layers of peat have been radiocarbon dated to give the approximate ages of the underlaying and overlying earthquake debris. In many areas through which the faults pass are terraces composed of gravel debris which has been washed down from the axial ranges. The ages of these terraces is known due to layers of dateable volcanic ash and wood preserved within. Some of these terraces have been offset by the fault. As the ages of these terraces is known it is then possible to tell how far the fault has moved in a given period of time.

The above studies have shown a record of at least 12M>6.5 earthquake events recorded on the Wellington Fault in the Woodville-Dannevirke district, 9 of which occurred in the last 30,000 years. This is the longest record of earthquake events recorded within a fault trench in New Zealand. The last earthquake on the Wellington Fault took place about 300 years ago between Dannevirke and Wellington. Horizontal offsets in the Dannevirke area were in the range of 10m and in the wellington area between 3 and 4m. The estimated magnitude of this earthquake was between 7.4 and 8.4 which would have caused major destruction of all larger buildings and engineering structures. Earthquakes of this size are estimated to occur every 300 to 400 years between Dannevirke and Wellington and every 300 to 500 years from the Ohara Depression north to the Napier-Taupo Highway. The Ruahine Fault (which is a branch of the Wellington Fault) between the Ohara Depression and the Napier-Taupo Highway is estimated to produce a 7.5 to 8 magnitude earthquake every 400 to 500 years. Horizontal movement is expected to be in the range of 3 to 5.5m. Dates for the last earthquake on the Ruahine Fault are not so accurate but it is possible that there have been up to 4 earthquakes on this fault since 1850 years before present.

Fortunately, except for the Wellington area these faults lie mostly on the eastern margin and within axial ranges. They pass mainly through farmland, areas of forestry and the Ruahine Range. It is possible that some farm houses in close proximity to the faults will receive damage but structures build on the fault may be ruptured or buried if in the path of any earthquake-triggered landslides. The large magnitude earthquakes can be expected to produce severe shaking in the cities of Palmerston North, Napier and Hastings.